<template>
  <div>
    <!--返回顶部-->
    <van-button v-if="btnFlag" icon="back-top" type="primary" @click="backTop" />
  </div>
</template>

<script>
export default {
  name: 'BackTop',
  data () {
    return {
      btnFlag: false // 默认按钮不显示
    }
  },
  // vue的两个生命钩子，这里不多解释。
  // window对象，所有浏览器都支持window对象。它表示浏览器窗口，监听滚动事件
  mounted () {
    window.addEventListener('scroll', this.scrollToTop)
  },
  // 页面离开，取消addEventListener，不然页面会报错
  destroyed () {
    window.removeEventListener('scroll', this.scrollToTop)
  },
  methods: {
    // 点击图片回到顶部方法，加计时器是为了过渡顺滑
    backTop () {
      const that = this
      const timer = setInterval(() => {
        const ispeed = Math.floor(-that.scrollTop / 5)
        document.documentElement.scrollTop = document.body.scrollTop = that.scrollTop + ispeed
        if (that.scrollTop === 0) {
          clearInterval(timer)
        }
      }, 16)
    },
    // 为了计算距离顶部的高度，当高度大于60显示回顶部图标，小于60则隐藏
    scrollToTop () {
      const that = this
      const scrollTop = window.pageYOffset || document.documentElement.scrollTop || document.body.scrollTop
      that.scrollTop = scrollTop
      if (that.scrollTop > 0) {
        that.btnFlag = true
      } else {
        that.btnFlag = false
      }
    }
  }
}
</script>

<style lang="less" scoped>
.van-button{
  width: 40px;
  height: 40px;
  border-radius: 50%;
  border: none;
  background-color: rgba(0, 0, 0, .5);
  position: fixed;
  right: 10px;
  bottom: 60px;
  z-index: 5;
}
</style>
